In the previous post, I've already defined the voicing rule as occurring in unstressed sequences of word-final *-VCᵊ#. However, I've also been pondering today on the possibility that the voicing here also involves laryngealization (aka creaky voicing). So now I will run some examples by you all to explain how this might explain not only word-final voicing in Late IE, but also the reflex of the thematic vowel, Szemerenyi's Law and maybe even the development of Narten presents. I know, I know, that's a lot on my plate but hear me out with a few examples showing the diachronic development:
Examples where voicing/laryngealization appears to occur:
*kahʷánasa 'dog (nom.)'Examples where voicing/laryngealization does not appear to occur:
*kwānz (> PIE *ḱwōn)
*ʔékwasa 'horse (nom.)'
*ʔékwəz (> PIE *h₁éḱwos)
*bérata '(s)he carried'
*[be:rt] (Syncope + Analogical Levelling with presentive 3ps *bḗrti)
*bērt (> PIE *bʰērt)
*nákʷtasa 'night (nom.)'
*[nakʷts] (Syncope; Lengthening blocked by consonant cluster; Assimilation)
*nakʷts (> PIE *nokʷts)
*kʷiz/*kʷid̰ (> PIE *kʷis/*kʷid)
*pad̰sa 'foot (nom.)'
*[pa:d̰z] (Syncope + Assimilation)
*pād̰z (> PIE *pōds)
*napátasa 'grandson (nom.)'
*[nə'patᵊsᵊ] (Reduction + Resistance)
*nepā́ts (> PIE *nepōts)
*bératai '(s)he carries'
*bḗrti (> PIE *bʰḗrti)
In the last example of the examples showing voicing/laryngealization, I show an instance of voicing in a completely unstressed word. The zerograde vowel *i in PIE *kʷid demonstrates that the word was unstressed during Syncope and this works well for the purposes of my new rule since apparently the preceding vowel is not as important as the vowel that follows it, which must be a supershort schwa. I presume that the difference in length between word-final supershort schwa and regular schwa is what triggered this change in the first place. The example of *h₁éḱwos 'horse' is meant to show the evolution of a thematic stem in the nominative. Its thematic vowel naturally then surfaces as *-o- because the nominative singular became voiced since the stage of Syncope. You may be skeptical of a voiced *-z contrasting with *-s in early Late IE (as I've been for some time) however it explains why PIE *-s is dropped in nominative singular *ḱwōn 'dog' while the *-s remains in the accusative plural ending *-ns. This can be solved by positing earlier *kwānz (from MIE *kahʷánasa with word-medial *-s-) and *-ns (from MIE *-am-as with word-final *-s) respectively. You may also note that certain vowels lengthen during Syncope in my above examples. I believe this is caused by the reduction of two supershort schwas in adjacent open syllables (i.e. *-V́C₁ᵊC₂ᵊ -> *-V̄́C₁C₂) via compensatory lengthening. This might then help to explain the rise of Narten presents which Jay Jasanoff considers to be most ancient. Personally, I don't believe that long vowels existed in the Mid IE period at all.
So whaddya say, people?! Isn't that dandy??? Sorry, it's getting me excited but all this is very complicated and making my head spin. I think this can work but we'll see what my fellow commenters say.
 Compare with RESTLE, D., ZAEFFERER, D., & VENNEMANN, T. (2002). Sounds and systems: Studies in structure and change: A festschrift for Theo Vennemann. Trends in linguistics, 141. Berlin, Mouton de Gruyter. p.24 (see link): "Since only word-internal tense plosives were preglottalized, the Old Danish apocope produced such pairs as WJut. kat - kat' 'cat - cats' [kat - kaʔt] corresponding to St.Da. kat - katte." It's not quite the same but still eerily similar to the PIE examples.
(Jun 16 2008) I found a sweet Danish parallel involving similar laryngealization just a few hours after I posted this entry which I've added as a footnote. Enjoy!